1. Field of the Invention
Aspects of the present invention relate to a power supply apparatus, and more particularly, to a switching mode power supply (SMPS) that prevents damage due to an over voltage or an over current.
2. Description of the Related Art
Generally, a switching mode power supply (SMPS) rectifies AC (alternating current) power inputted from the outside, to be supplied to elements of an electronic device. The SMPS is widely employed in electronic devices, such as a printer.
FIG. 1 is a circuit diagram of a conventional switching mode power supply (SMPS). The conventional SMPS includes a transformer T1, a photocoupler light emitter PD1 and a photocoupler light receiver PT1, a capacitor C1, a control IC, and a switching element Q1. The transformer T1 induces input power converted into DC power by a rectifier to a secondary winding. The photocoupler light emitter PD1 and the photocoupler light receiver PT1 respectively emit and receive light by the induced power of the secondary winding. The capacitor C1 is connected with the photocoupler light receiver PT1 in parallel. The control IC detects a voltage of the capacitor C1. The switching element Q1 is switched by the control IC.
The conventional SMPS limits output power (an output voltage multiplied by an output current of the secondary winding of the transformer T1), within a proper level. Over current protection (OCP) and over voltage protection (OVP) systems can limit the output power. As shown in FIG. 1, if an output current or an output voltage becomes too high compared to a reference value, a control IC controls the switching element (Q1) to stop switching, thereby limiting the output power.
In some cases, however, the output voltage or the output current may remain between the reference value at which the switching operation is suspended and a normal value (the SMPS may operate in an abnormal state). Under this circumstance, if the output power is not properly limited, a continued over voltage or over current may damage the power supply apparatus and a circuit connected to an output terminal of the power supply apparatus.
Thus, if an abnormal over voltage is continuously applied to the secondary winding of the transformer T1 as shown in FIG. 1, the photocoupler light emitter PD1 generates and transmits a light signal to the photocoupler light receiver PT1. A current flows to the photocoupler light receiver PT1, thereby influencing a voltage of the capacitor C1 connected thereto in parallel. The control IC detects the voltage of the capacitor C1 and changes a duty ratio of a pulse width modulation (PWM) signal applied to the primary winding of the transformer T1, based on the detection result. The control IC performs feedback by controlling an output voltage. However, the conventional SMPS does not properly control the continued abnormal current while limiting the output power if the abnormal voltage is continuously applied. A circuit can be damaged if the over current is continuously applied for a long time.